Fluid mixing system

ABSTRACT

A method for continuously mixing a borehole fluid such as cement includes using a measurement of the solid fraction of a cement slurry as it is being mixed to determine the ratio of the solid and liquid components to be added to the slurry. A system for mixing the includes a liquid material (water) supply including a flow meter; a solid material (cement) supply; a mixer which receives the liquid and solid materials and includes an output for delivering materials from the mixer to a delivery system; a device for measuring the amount of material in the mixer; and a flow meter in the output; wherein measurements from the flow meters and the device for measuring the amount of material in the mixer are used to control the amount of solid and/or liquid material added to the mixer.

The present invention relates to a system for mixing fluids containingsolid and liquid materials such as cement. In particular the inventionprovides a system for the continuous mixing of cements or other fluidsused in the drilling, completion or stimulation of boreholes such as oilor gas wells.

When a well such as an oil or gas well has been drilled, it is oftendesired to isolate the various producing zones form each other or fromthe well itself in order to stabilise the well or prevent fluidcommunication between the zones or shut off unwanted fluid productionsuch as water. This isolation is typically achieved by installing atubular casing in the well and filling the annulus between the outsideof the casing and the wall of the well (the formation) with cement. Thecement is usually placed in the annulus by pumping a slurry of thecement down the casing such that it exits at the bottom of the well andpasses back up the outside of the casing to fill the annulus. While itis possible to mix the cement as a batch prior to pumping into the well,it has become desirable to effect continuous mixing of the cement slurryat the surface just prior to pumping into the well. This has been foundto provide better control of cement properties and more efficient use ofmaterials.

The cement slurries used in such operations comprise a mixture of dryand liquid materials. The liquid phase is typically water and so isreadily available and cheap. The solid materials define the slurry andcement properties when added to the water and mixed, the amount of solidmaterials in the slurry being important. Since the liquid phase isconstant, the amount of solid material added is usually monitored bymeasuring the density of the slurry and maintaining this at the desiredlevel by controlling the amount of the solid material being added. FIG.1 shows a schematic diagram of a prior art mixing system. In the systemof FIG. 1, mix water is pumped from a feed supply 10 via a pump 12 to amixer 14 which feeds into a mixing tub 16. The feed supply 10 comprisesa pair of displacement tanks 11, 11′ each with separate outletsconnected to a valve 13 which in turn feeds the pump 12. Two methods arecommonly used to determine the amount of water supplied:

-   1. Proximity switches installed on the shaft of the pump 12 count a    number of pulses per rotation. Each pulse corresponds to a    displacement volume. This method is sensitive to pump efficiency.-   2. Displacement volume is measured by counting the number of tanks    pumped down-hole. This measurement method is sensitive to human    error in level reading, switching from on tank to another and tank    exact capacity. Even more an error in the number of tanks counted    can have many consequences (over displacement can result in wet    shoe, under displacement can result in no pressure bump or cement    left in the casing).

Solid materials are delivered to the mixer 14 from a surge can 18 ordirectly from a cement silo via a flow control valve 20 and are carriedinto the mixing tub 16 with the mix water. The contents of the mixingtub 16 are recirculated through a recirculation pipe 22 and pump 24 tothe mixer 14. The recirculation pipe 22 also includes a densitometer 26which provides a measurement of the density of the slurry in the mixingtub 16. An output 28 is provided for slurry to be fed from the mixingtub 16 to further pumps (not shown) for pumping into the well. Controlof the slurry mixture is achieved by controlling the density in themixing tub 16 as provided by the densitometer 26 by addition of solidmaterial so stay at a predetermined level for the slurry desired to bepumped. The densitometer 26 is typically a non-radioactive device suchas a Coriolis meter.

While this system is effective for slurries using materials of muchhigher density than water, it is not effective for slurries using lowdensity solid materials, especially when the density of the solids isclose to that of water. In such cases, a density measurement is notsensitive enough to control the amounts of solid material added to thenecessary accuracy.

The present invention seeks to provide a mixing system which avoid theproblem of density measurement described above.

In its broadest aspect, the present invention comprises using ameasurement of the solid fraction of a fluid as it is being mixed todetermine the ratio of the solid and liquid components added to theslurry.

The invention is particularly applicable to the mixing of boreholecement slurries, in which case, solids fraction is determined as (slurryvol−water vol)/slurry vol. An alternative but related parameter isporosity, determined as water vol/slurry vol (porosity+solidsfraction=1).

A system for mixing cement in accordance with the invention comprises aliquid (water) supply including a device for measuring the amount ofliquid supplied; a solid material supply; a mixer which receives theliquid and solid materials and includes an output for deliveringmaterials from the mixer to a delivery system; a device for measuringthe amount of material in the mixer; and a flow meter in the output;wherein measurements from the flow meters and the device for measuringthe amount of material in the mixer are used to control the amount ofsolid material added to the mixer.

The flow meters can be mass flow meters or volumetric flow meters. Anysuitable form of meter can be used, for example Coriolis meters orelectromagnetic meters.

The mixer will typically include a tank or tub, in which case the devicefor measuring the amount of material in the mixer can be a level sensor.Such a level sensor is preferably a time domain reflectometry- orradar-type device although acoustic or float devices can also be used.It is preferred to mount such a device in an arrangement for dampingtransient fluctuations in the tank level, for example in an arrangementof concentric slotted tubes. An alternative or additional form of sensorcan be a load cell which can be used to indicate the weight of the tank,or a pressure sensor.

The device for measuring the amount of liquid supplied can be a flowmeter or a level sensor of the types described above. When the liquidsupply includes one or more displacement tanks, a level sensor ispreferred

Where the mixer includes some form of recirculation of the slurrythrough the tank, it is important that the output flow meter isdownstream of this recirculation.

Where the solid materials comprise cement and other solid additivesadded separately to the mixer, separate flow meters can also be providedfor each separate supply of additives.

In its simplest form, the measurement of solid fraction is used as aguide for the operator to add solids, particularly cement, to the slurryas it is mixed. In more advanced versions, the calculation of solidsfraction is used to control the addition of solids directly by means ofan automatic control system.

The invention also provides an improved method for calculatingdisplacement volume from a system comprising at least one displacementtank, comprising measuring the level of liquid in the tanks over timeand calculating displacement as:Σ(V(h1n)−V(h2n))where:

-   V(h) is the exact volume of the tank at level (h);-   h1n is the start level of the n th displaced tank volume; and-   h2n is the stop level of the n th displaced tank volume.

Examples of the present invention will now be described with referenceto the accompanying drawing, in which:

FIG. 1 shows a prior art mixing system;

FIG. 2 shows a mixing system according to a first embodiment of theinvention;

FIG. 3 shows the components of a tank level sensor;

FIG. 4 shows the components of the level sensor assembled;

FIG. 5 shows a schematic of the tank level measurement; and

FIG. 6 shows a mixing system according to a second embodiment of theinvention.

The system shown in FIG. 2 is used for the continuous mixing of cementfor oil well cementing operations and comprises a supply of mix water100 feeding, via a pump 102 and a flow meter 104 to a mixing system 106.

The supply of mix water comprises a pair of displacement tanks 101, eachhaving a separate output connected to a valve 103 which supplies thepump 102. Level sensors 105 are included in each displacement tank 101for determining the amount of water supplied to the pump 102. In anotherversion (not shown), the level sensors are omitted. The amount of watersupplied is determined in the manner described below.

The mixing system 106 also receives solid materials from a surge can 108(or alternatively directly from a surge can) which are admitted througha valve 110. The mixed solid and liquid materials are delivered througha feed pipe 112 to a mixing tub 114. The mixing tub 114 has a firstoutlet 116 connected to a recirculation pump 118 which feeds the slurrydrawn from the tub 114 back into the mixing system. The tub 114 isprovided with a level sensor 120 and/or a load sensor 122 to provide anindication of the tank contents and any change in contents over time. Asecond output 124 is provided from the tub 114 which leads, via a secondpump 126 and a second flow meter 128 to the pumping system from which itis delivered to the well (not shown). An alternative method of delivery(shown in dashed line in FIG. 2) has an output 124′ taken from therecirculation line via a flow meter 128′ to the well. Other arrangementsare also possible. The pumps 102, 118, 126 are of the usual type foundin well cementing systems, for example centrifugal pumps. Likewise, theflow meters 104, 128′ are conventional, for example Coriolis meters suchas those that have been used as densitometers in previous applications.Different types of pumps and meters each have advantages anddisadvantages that are well known in the art and can be selectedaccording to requirements.

FIGS. 3-5 show details of the level sensors used in the displacementtanks and tub and the manner of installation. The sensor comprises aKrohne radar sensor 200, a stainless steel rod 202, an inner slottedsleeve 204 and an outer slotted sleeve 206. The rod 202 is screwed ontothe sensor 200 and the inner sleeve 204 mounted over the rod 202 andattached to a flange on the sensor 200. The outer sleeve 206 is mountedover the inner sleeve 204 to which it is attached. 28 For use in thedisplacement tanks, each displacement tank receives a level sensor. Thissensor gives an accurate measurement of the liquid level in the tank.The exact volume versus level is required to calculate the displacedvolume. In case the tank cross section profile is not accurately known aso-called tank calibration is performed. A water meter equipped with adigital output measures the exact displacement tank volume versus tanklevel. This operation is performed only once for each tank. To supplywater to the system, the valve 103 is operated to allow water to flowfrom one or other tank to the pump 102. When a tank discharge valve isopened, a device such as an end switch, pressure switched or any otherappropriate device is used to begin calculation of the displacementvolume. Displacement volume is then computed as:Σ(V(h1n)−V(h2n))Where:

-   V(h) is the exact volume of the tank at level (h)-   h1n start level of the n th displaced tank volume-   h2n stop level of the n th displaced tank volume    When the level in the tank in use becomes low, the supply is    switched to the other tank.    Switching operation from one tank to another can either be manual or    automated and when one tank is emptying the other one is filled up    for further use. Since the level sensors can be used to give an    instantaneous measurement of the amount of water provided to the    system, it is possible to confirm the data provided by the flow    meter 104, or even to replace the need for this flow meter    completely. When the flow meter is present, it is not essential to    have the level sensors in the displacement tanks.

This method of determining the displacement volume can be applied toother forms of cementing operation than the ones described here, and hasthe advantage that it is relatively insensitive to pump efficiency oroperator error as found in the previous systems.

For use in the mixing tub, the sensor arrangement is installed in themixing tub 114 in the vertical position and in a location where theslurry is renewed as the mixing occurs, to avoid location in a dead zonewhere cement might set. The sensor provides a measurement of thedifference between the length of the rod 202 (LM) and the level ofslurry in the tub level (TL). The free tub level (FTL) is obtained by:FTL=LM−TL.It will be appreciated that the exact form of level sensor is notimportant to the overall effect of the invention. What is important isto obtain an indication of the variation versus time of the tub slurryvolume (called “tub flow” in this document). This can be obtained usinga float or a load sensor or combinations of any of these or any othersensor giving this information.

The outputs of the flow sensors and level sensors are used to monitorthe solid fraction of the slurry in the following manner:

The solid fraction computation is based on the balance between incomingand outgoing volumes (or flow rates) as expressed in the followingrelationship:Q _(water) +Q _(cement) =Q _(slurry) +Q _(sub)where Q_(tub) is the tub rate.

Tub rate is the variation versus time of the tub volume and isconsidered as positive while the tub level increases and negative whileit decreases. The smaller the tub cross section, the more sensitive themeasurement will be to change. Q_(tub) is given by:$Q_{tub} = {S_{tub}\frac{\mathbb{d}h_{tub}}{\mathbb{d}t}}$where S_(tub) is the tub cross section and$\frac{\mathbb{d}h_{tub}}{\mathbb{d}t}$is the tub level variation over time. In the simplest case, the tubsection is constant and the tub rate becomes the product of the tublevel variation/time and the tub cross section.

The solids fraction at time t is computed as the ratio of (slurryvol−water vol) over the total slurry volume present at time t in thetub. The variation in tub slurry volume V_(tub)(t+δt)−V_(rub)(t) can beexpressed as:V _(tub)(t+δt)−V _(tub)(t)=└Q _(water)(t)+Q _(cement)(t)−Q_(slurry)(t)┘*δtwhich can be rewritten as:V _(tub)(t+δt)−V _(tub)(t)=Q _(tub)(t)* δt.In the same way, the variation in the water volume present in the tub attime t V_(water)(t+δt)−V_(water)(t) is equal to the incoming watervolume minus the amount of water present in the slurry leaving the tub,and can be expressed as:V _(water)(t+δt)−V _(water)(t)=└Q _(water)(t)−(1−SolidFraction(t))*Q_(slurry)(t)┘*δt.Solid Fraction is then expressed as:${{SolidFraction}\left( {t + {\delta\quad t}} \right)} = {1 - \frac{{V_{water}(t)} + {\left\lfloor {{Q_{water}(t)} - {\left( {1 - {{SolidFraction}(t)}} \right)^{*}{Q_{slurry}(t)}^{*}}} \right\rfloor^{*}\delta\quad t}}{{V_{tub}(t)} + {{Q_{tub}(t)}^{*}\delta\quad t}}}$The calculation requires that the initial conditions be known if it isto be accurate ab initio, i.e. is the tub empty, full of water orcontaining slurry already. The calculation will ultimately stabiliseindependently of the initial conditions, the time taken to do thisdepending on the tub volume and the output flow rate Q_(slurry).

These calculations are conveniently performed using a computer, in whichcase the measurements can be provided directly from the sensors via asuitable interface. A preferred screen display will show the variousflow rates or levels, together with the desired solids fraction(calculated when designing the slurry). The mixing process is controlledby adjusting the amount of cement and/or water added to the mixer so asto maintain the calculated solids fraction at the desired level.Alternatively, the results of the calculations can be fed to anautomatic control system which adjusts the rate at which the componentsare delivered to the mixing system.

The system described above works well when the dry ingredients (blend ofcement+additives) are delivered pre-mixed to the well site from anotherlocation. In this case essentially the same measurements andcalculations as described above are performed, merely substitutingQ_(blend) for Q_(cement). If it is desired to mix the dry materials onsite as part of the continuous mixing process, a slightly differentapproach is required. FIG. 6 shows a mixing system according to anotherembodiment of the invention and uses a numbering scheme which followsthat of FIG. 2. The system of FIG. 6 comprises an additional drymaterial supply 130 which admits the dry products to the mixing system106 via a mass flow meter 132 (other flow measurement means can also beused) and a control valve 134. In this case, the basic control equationbecomes:Q _(water) +Q _(additive) +Q _(cement) =Q _(tub) +Q _(slurry)where four of the five variables are know and Q_(cement) is the mostdifficult parameter to measure accurately. Where multiple dry additivesare to be added, the supply can comprise separate material supplies,each with a flow meter and valve. Additional terms Q_(additive1),Q_(additive2), etc., are included in the control equation.

It will be appreciated that changes can be made in implementation whilestill remaining within the scope of using solid fraction as the propertymonitored to effect control of the mixing.

For example, the method can be applied to the mixing of other boreholefluids such as stimulation fluids (fracturing fluids) or even drillingfluids (mud). In the case of fracturing fluids, the gel and proppant(liquid and solid phases) are usually mixed using a pod blender and theproportion of gel and proppant controlled using a densitometer (usuallyradioactive) downstream of the mixer/blender. The use of radioactivesensors generates many environmental issues and while Coriolis-typemeters are an alternative, they are know to have limitations in respectof flow rate when used this way. The present invention allows control ofproppant and gel concentrations by means of flow meters without the needto rely on densitometer measurements.

Gel and mixed fluid flow rates are measured by means of electromagneticflow meters. The amount of proppant is directly deduced from thefollowing relationship:Q _(gel) +Q _(Pr oppant) =Q _(MixedFluid)Proppant concentration (in Pounds Per Gallon Added or “PPA”) can be afunction of solid fraction as defined above and expressed as thefollowing:PPA=Proppant Density*Solid Fraction/(1−Solid Fraction).Thus the solid fraction measurement methodology described above inrelation to cement can be applied to fracturing fluids by determiningproppant density rather than cement density.

This approach has the advantage of not requiring the use of radioactivedensitometers thus avoiding limitations placed on use for regulatoryreasons and without the flow rate performance limitations of othermeasurement techniques. The equipment and control system is essentiallythe same as that used in the cementing system described above.

1-17. (canceled)
 18. A method of mixing solid and liquid components in amixer to form a slurry, the mixer comprising a liquid input, a solidinput, and a slurry output, the method comprising: i) measuring at leastone flow rate of the group selected from the flow rate of the liquidcomponent provided through the liquid input into a mixer, the flow rateof the solid component provided through the solid input into the mixer,and the flow rate of the slurry out of the mixer through the slurryoutput; ii) measuring the amount of slurry in the mixer; iii) using theflow rate measurement and the slurry measurement to calculate the solidfraction or the porosity of the fluid; and (iv) controlling the supplyof the liquid component or the supply of the solid component to themixer according to the calculated solid fraction or porosity.
 19. Themethod of claim 18 wherein at least one of the solid and liquidcomponents is continuously provided to the mixer, and the slurry iscontinuously removed from the mixer.
 20. The method of claim 18 whereinadditives are delivered to the mixer separately from the solidcomponent, the method further comprising measuring the flow rate ofadditives delieverd to the mixer.
 21. The method of claim 18 wherein themixer includes a tub, the measurement of the amount of slurry in themixer comprising a measurement of the amount of slurry in the tub. 22.The method of claim 18 wherein a portion of the slurry is recirculatedinto the mixer as solid components or liquid components are added. 23.The method of claim 22 wherein the recirculation takes place upstream ofthe measurement of the flow rate of slurry removed from the mixer. 24.The method of claim 18 wherein the slurry is a borehole fluid.
 25. Amethod of mixing a slurry comprising: i) delivering a solid componentand a liquid component to a mixer, the mixer comprising a liquid input,a solid input, and a slurry output; ii) mixing the solid component andthe liquid component to form a slurry; iii) measuring the amount ofslurry in the mixer; iv) measuring at least one flow rate of the groupselected from the flow rate of the liquid component provided through theliquid input into a mixer, the flow rate of the solid component providedthrough the solid input into the mixer, and the flow rate of the slurryout of the mixer through the slurry output; v) calculating the solidfraction or porosity of the slurry using the flow rate measurement andthe slurry amount measurement.
 26. The method of claim 25 wherein thestep of measuring the amount of slurry in the mixer comprises measuringthe level of the slurry in the mixer.
 27. The method of claim 25 furthercomprising: vi) adjusting the flow rate of the liquid component to themixer according to the calculated solid fraction or porosity of theslurry.
 28. The method of claim 25 further comprising: vi) adjusting theflow rate of the solid component to the mixer according to thecalculated solid fraction or porosity of the slurry.
 29. The method ofclaim 25 wherein the step of delivering further comprises continuouslydelivering at least one of the solid component and the liquid componentto the mixer.
 30. The method of claim 25 wherein the step of calculatingthe solid fraction or porosity of the slurry is performed using acomputer.
 31. The method of claim 15 wherein the slurry is cementslurry.
 32. A method of mixing a slurry wherein a solid component and aliquid component are delivered to a mixer and the slurry is removed fromthe mixer, the method comprising: i) measuring the flow rate of at leastone of the solid component and the liquid component into the mixer; ii)measuring the flow rate of slurry removed from the mixer; iii) measuringthe level of the slurry in the mixer; iv) calculating the solid fractionor porosity of the slurry using a measurement of flow rate and theslurry level measurement; and (v) controlling the delivery of solidcomponent or liquid component to the mixer according to the calculatedsolid fraction or porosity.
 33. The method of claim 32 wherein theslurry is a borehole fluid.
 34. The method of claim 33 when the slurryis cement slurry.
 35. The method of claim 32 wherein steps i) throughiv) are repeated over time.
 36. The method of claim 33 when the slurryis stimulation fluid.
 37. The method of claim 33 when the slurry isdrilling fluid.